In a mobile communication system supporting LTE (Long Term Evolution)-Advanced, CA (Carrier Aggregation) can be performed by using a Pcell (Primary Cell) which is a serving cell in a PCC (Primary Component Carrier) and an Scell (Secondary Cell) which is a serving cell in an SCC (Secondary Component Carrier).
Meanwhile, such a mobile communication system supporting LTE-Advanced is configured to undergo TA (Timing Advance) control.
Specifically, in the TA control, as shown in FIG. 8, in order to synchronize reception timings (FFT timings) of reception signals from multiple mobile stations UE#1 and UE#2, a radio base station eNB is configured to inform the mobile stations UE#1 and UE#2 of TA values, while the mobile stations UE#1 and UE#2 are configured to adjust transmission timings of uplink signals on the basis of the informed TA values.
For example, as shown in FIG. 8, the radio base station eNB causes the mobile station UE#2 located on a cell edge to transmit an uplink signal at an earlier timing than that from the mobile station UE#1 located at a cell center.
On the other hand, when “Inter-band UL CA” is performed by using CCs (Component Carriers) of different bands, radio characteristics (delay characteristics and attenuation characteristics) vary among the CCs. Here, it is necessary to carryout the TA control of cells in the CCs having the different radio characteristics in such a manner as to conform to the individual radio characteristics.
In addition, when each CC has a different transmission point, or when “UL CA” is performed by using a CC#1 and a CC#2 with the CC#1 being under the control of a macro radio base station and the CC#2 being under the control of a small radio base station (for example, an RRH), for instance, it is necessary to carry out the TA control in conformity to respective propagation delays in that case as well because there is a difference between the distances to a mobile station UE from the radio base stations eNB.
Note that the above-mentioned situation occurs not only in the case of performing “Inter-band UL CA” but also in the case of performing “UL CA” in general.
Specifically, when “UL CA” is performed, the mobile communication system supporting LTE-Advanced is configured to group a Pcell and Scells #1 to #3 into groups (TAGs; Timing Advance Groups) each having equivalent radio characteristics as shown in FIG. 9, and to carry out the TA control for each of the groups.
The above-described TA control is called “Multiple Timing Advances (MTA) control.”
Here, there are two types of groups, namely, a pTAG including the Pcell (a TAG#A in the example of FIG. 9) and an sTAG consisting of the Scells (a TAG#B in the example of FIG. 9).